Machine tool position servo including a linear adjustable cam for compensating lead-screw error



Dec. 31, 1968 J. 0. BLACK ET L 3,419,773

MACHINE TOOL POSITION SERVO INCLUDING A LINEAR ADJUSTABLE A CAM FOR COMPENSATING LEAD-SCREW ERROR Filed Jan. 27, 1965 l Lu:

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JAMES BNgENTORS CHARLES awak HAM BY M'D'M THEIR ATTORNEY United States Patent 3,419,773 MACHINE TOOL POSITION SERVO INCLUDING A LINEAR ADJUSTABLE CAM FOR COMPENSAT- ING LEAD-SCREW ERROR James 1). Black and Charles H. Winterbotham, Dayton,

Ohio, assignors to The Bendix Corporation, a corporation of Delaware Filed Jan. 27, 1965, Ser. No. 428,435 6 Claims. (Cl. 31818) ABSTRACT OF THE DISCLOSURE A device is disclosed for correcting a displacement signal to accurately conform to a corresponding actual displacement. A cam having an adjustable surface is carried on a slide of a machine which is driven by means such as a lead-screw and motor. A signal transducer provides an indication of distance travelled. A cam follower is attached to the transducer such that the transducer is rotated in response to the cam surface, thus adjusting or compensating the signal.

This invention pertains to a displacement error compensating device and more particularly to a device for correcting a displacement signal to accurately conform to a corresponding actual displacement.

One object of this invention is the provision of a displacement error compensating device which is flexible in application, of simple construction, economical to manufacture, and efficient in operation.

Another object of this invention is the provision of such a compensating device including cam means for use in correcting a displacement signal to accurately conform to a corresponding actual displacement, which cam means is readily adjustable as to contour to compensate for defects in the system which achieves and signals displacement as determined in initial setup and occurring through wear, misuse, etc. during operation.

Another object of this invention is the provision of such a compensating device including precision cam means of the character mentioned comprised of a series of relatively inexpensive mechanical elements arranged side by side to form a cam surface which is capable of being readily adjusted to suit individual requirements and varying conditions.

Another object of this invention is to provide such a device wherein both signal and drive means are coaxially arranged along a common shaft thus achieving the utmost in rigidity and positive coupling, eliminating errors and deficiencies occurring through backlash, lost motion and the like.

Other objects and advantages of the invention will be apparent from the following description, the appended claims, and the accompanying drawing, in which,

FIG. 1 is a perspective view of a measuring machine which incorporates an exemplary embodiment of the invention,

FIG. 2 is a view with parts in section and parts broken away showing the exemplary application of the compensating device of this invention adapted to the machine of FIG. 1,

FIG. 3 is a section on the line 33 of FIG. 2, and

FIGS. 4 and 5 are side and end views respectively with parts in section and parts broken away of the compensating device showing individual adjustments for each cam element.

In the illustrated embodiment of this invention a displacement error compensating device is applied in a measuring machine and shown in FIG. 1. Machine 10 has a base 11 which carries a slide 12 for reciprocating movement back and forth thereon. Slide 12 supports a workpiece 13 which is contacted by a gaging probe 14 carried by an upright portion 15 of base 11.

The displacement error compensating device illustrated in this exemplary embodiment is associated with horizontal movements and includes relatively displaceable first and second members such as base 11 and slide 12. Drive means, including motor 16 (FIG. 2) driving a rotatable screw 17, is provided for relatively displacing slide 12 on base 11 and signal means operated by said drive means provide a displacement signal in response to the relative displacement of said members. However, such displacement signal may not be accurately representative of the true displacement between said members, i.e. the movement of slide 12 on base 11 due to, for example, variations in the lead along the length of screw 17. To solve this problem a compensating cam 20 having an adjustable cam surface contour 21 is provided. A cam follower 22 is operatively connected between said first and second members (slide 12 and base 11) for compensating movement therebetween during relative displacement of slide 12 on base 11. With slide movement cam follower 22 cooperates with the adjustable cam surface 21 of cam 20 and provides compensating corrections to the displacement signals from said signal means as will be subsequently described.

Compensating cam 20 is comprised of a plurality of discrete elements 23 supported by a U-shaped channel 18 and which are individually adjustable to form compensating contour 21. Cam 20 is carried by slide 12 and preferably fixed thereto and moves back and forth therewith.

Means is provided for readily individually adjusting each discrete element 23 comprising cam surface 21 as illustrated in FIGS. 4 and 5. Such adjustment effectively modifies the compensating contour of cam surface 21 and includes a bolt 30 for each element threaded through the bottom of U-shaped channel 18 which supports and positions a corresponding element 23 vertically. A set screw 32 is provided for each bolt 30 to lock each such bolt into position. Each set screw 32 is threaded through a wall of channel 18 and then into engagement with a corresponding bolt 30. Another bolt 33 is provided for each corresponding element 23 to position such element against lateral movemnts. Each bolt 33 is threaded through a side of channel 18 and into engagement with a corresponding element 23 to firmly hold such element in position against the opposite side of channel 18.

A double roller 24 supports cam 20 throughout its full length during movement of slide 12 back and forth on base 11 while enabling essentially frictionless movement. The length of cam 20 corresponds to the full travel of slide 12 and also corresponds to the effective length of drive screw 17 utilized in the drive means. While compensation is required for extreme accuracy, with precision lead screws the variation in upward projection of adjacent elements is such that the cam contour is effectively continuous. The bottom of U-shaped channel 18 of cam 20 is in direct rolling contact with double roller 24 and supported thereby.

Cam follower 22 is carried at one end of an arm 25 which, as illustrated in FIG. 2, is supported to move in and out of the plane of the drawing. The opposite end of arm 25 is operatively connected to the signal means to provide compensating corrections as will be later described. Cam follower 22 is arranged to engage cam surface 21 at a position generally opposite where cam 20 is supported by roller 24, see FIG. 4, thereby minimizing any tendency for cam 20 to be deflected by the pressure from follower 22 and thus introduce signal errors. Cam follower 22 is yieldingly urged toward adjustable cam surface 21 by a spring 26 carried in a housing member 27 suitably fastened to base 11.

As previously mentioned, the drive means for relatively displacing slide 12 on base 11 preferably includes a rotatable ball screw 17 supported for rotation on base 11. Screw 17 threadedly engages a drive nut, preferably a ball nut 35, fixed to slide 12 by bolts passing through holes 36a in a flange 36 of such ball nut and threaded into table 12. Rotation of screw 17 drives nut and slide 12 back and forth on base 11. Screw 17 has unthreaded shaft portions or journals 37 and 38 at its opposite ends which are carried respectively by bearing assern'blies 39 and 40 mounted on base 11. Ball screw 17 has an effective threaded length which of course determines the movement of slide 12 and as previously mentioned also determines the effective length of cam 20. An extension shaft portion 41, which of course is a rotatable drive member, extends coaxially from screw 17 beyond journal 37.

The drive means in this example preferably employs electric energy for driving motor 16 and thus rotate screw 17 which in turn moves slide 12 on base 11. Electric motor 16 has an armature 43 fixed to extension 41 and a field which is mounted on base 11. It will be seen that this structure provides the utmost in rigidity and positive coupling because the drive screw and motor armature are essentially the same structural unit, thus eliminating errors and deficiencies occurring through backlash, lost motion, and the like.

A further extension of shaft portion 41 is illustrated at 44 and engages a coupling member 45 provided to take deflections due to misalignment of the components while transmitting rotary displacements without error.

The signal means is operated by said drive means to provide electrical output displacement signals in response to the relative displacement of slide 12 on base 11 as produced by motor 16 driving screw 17. The signal means includes transducer means such as transducer 48 having first and second relatively movable displacement signal producing components comprising a relatively rotatable shaft 46 coupled to screw 17 and housing 47 coupled to cam follower 22.

Transducer 48 receives such relative rotation of shaft 46 with respect to housing 47 and converts such relative rotation into an equivalent electrical output signal. The transducer means 48 may be an electromechanical device of any known form in which a rotary mechanical signal is converted to an equivalent electrical signal.

Means directly coupling shaft 46 coaxially with rotatable drive member or shaft portion 41 includes a bore 51 in one end of the end wall of member 45. Shaft 46 is suitably fastened in bore 51 such that rotation of screw 17 transmits rotary mechanical motion to shaft 46. It will be seen that this direct coupling of drive screw 17 with transducer 48 in a coaxial manner through a common shaft, eliminates error producing complexity and enables correction of defects in both the mechanical and the electrical operating components by merely adjusting a single compensating contour 21 through adjustment of individual elements 23.

Means mounting housing 47 on base 11 for limited rotary adjustment includes a bearing assembly 52 comprised of a member 53 mounted on base 11 which supports the outer race of bearing 52 while its inner race carries an annular member 54 for rotation therewith. Housing 47 is suitably fastened to member 54 as by screws 55. As previously mentioned the relative rotation of shaft 46 with respect to housing 47 determines the electrical output signal provided by transducer 48.

Arm 25 is also fastened to member 54 such that as cam follower 22 moves across cam surface 21 the amount of relative rotation of shaft 46 with respect to housing 47 is modified, thus modifying the electrical output signal. In this embodiment of the invention arm 25 has an L shape for ease of mounting with the other cooperating components, also cam follower 22 has a roller for contacting surface 21 and thus enable friction free movement.

In setting up the system for operation, drive motor 16 is energized and the signal from transducer 48 is displayed on a console 56 to indicate the displacement of slide 12 on base 11. Through use of an independent measuring device the actual true displacement, from a desired reference point for example, would be determined precisely. Elements 23 would then be individually adjusted to correct the amount of rotation of arm 25 and hence the relative rotation of shaft 46 with respect to housing 47 until the indicated displacement corresponded to the actual displacement independently determined. This procedure would be repeated along the full length of travel of slide 12. It will be obvious that with this technique it is immaterial whether errors are introduced by either the mechanical or electrical system or both in an abrupt, cumulative, uniform, or random manner. Such errors will be compensated for along the full length of travel of slide 12.

The individual elements 23 making up the compensating surface 21 of cam 20 are solid blocks preferably in the form of metallic parallelepipeds which are relatively inexpensive to manufacture and which can be readily assembled and adjusted in any desired contour to either side of an arbitrary reference plane surface by merely adjusting the screws which position them. The reference surface could, for example, represent a theoretically perfect system in which no compensation is required. Such surface would be formed in this embodiment by the top surface (FIG. 4) of each element 23. The size of each individual element 23, or the number employed, is not critical; however, in one preferred embodiment of this invention each element 23 corresponds exactly in length, along the direction of slide movement, to the pitch of drive screw 17, the axis of which lies parallel to such movement.

Thus it will be seen that a system is provided for accurately establishing the displacement between relatively movable members which include a versatile adjustable compensating device which is economical to manufacture and efficient in operation. Such a system makes it possible to achieve the utmost in rigidity and positive coupling between drive and signal components while providing a device which through adjustment of a single compensating contour eliminates errors and deficiencies in all operating components of the system. The adjustable device of this invention offers a rapid means of obtaining the proper compensating contour in the initial setup without manufacturing delays, and also offers full flexibility to readily accommodate variations which later occur in operation-such as those caused by uneven wear of the driving components or slides.

While the form of apparatus herein described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. A displacement error compensating device for use in a machine comprising:

abase,

a slide carried for movement back and forth on said base in a rectilinear path,

a drive nut fixed to said slide movement therewith,

a rotatable screw mounted for rotation on said base and in threaded engagement with said nut, said screw having an extension shaft portion extending coaxially therefrom,

electric motor drive means having an armature fixed to said extension shaft for rotation therewith and for rotating said screw and thus move said nut and slide back and forth on said base,

signal means including transducer means providing an electrical output displacement signal in response to the relative displacement of said slide on said base and having a relatively rotatable shaft and housing,

means directly connecting said shaft coaxially with said rotatable drive member to receive a rotary mechanical input motion therefrom,

means mounting said housing on said base for limited rotary adjustment with respect to said shaft thus modifying the relative rotation of said shaft and housa compensating cam carried by said slide for movement therewith and having an adjustable cam surface comprised of a plurality of discrete elements aligned rectilinearly along the direction of movement and carried for individual adjustment transverse said direction,

a cam follower carried by said base and arranged to operatively engage said cam surface and receive a limited rotary adjustment therefrom,

means connected to said cam follower at one end to receive said rotary adjustment and connected at the opposite end to said housing imparting limited rotary corrective motion thereto to modify said relative rotation and thereby modify said electrical output signal,

and means for readily adjusting said discrete elements individually transverse said direction of movement to thus modify the effective compensating contour of said cam surface so that said rotary adjustment corrects the relative rotation of said shaft and housing and provides displacement signals accurately representative of the displacement between said members while compensating for errors in said drive and signal means.

2. A device as set forth in claim 1 in which said compensating cam is mounted parallel to the axis of said screw wherein said cam follower is coaxially rotatable.

3. A device as set forth in claim 1 further including means urging said cam follower against said cam surface with a constant pressure providing a constant engagement between said follower and cam surface for accurate relative movement therebetween.

4. A device as set forth in claim 3 further including means acting on said compensating cam in opposition to said follower thereby minimizing any tendency for said cam to be deflected by the pressure from said follower.

5. A device as set forth in claim 1 in which said compensating cam further comprises a channeled member supporting said discrete elements wherein one side of said elements is exposed for contact by said follower, and independent mechanical adjustment means engaging the opposite side .of each of said elements.

6. A displacement error compensating device for use in a machine comprising, a base;

a slide carried for movement back and forth on said base in a rectilinear path,

a drive nut fixed to said slide for movement therewith,

a rotatable screw mounted for rotation on said base and in threaded engagement with said nut, said screw having an extension shaft portion extending coaxially therefrom,

electric motor drive means having an armature fixed to said extension shaft for rotation therewith and for rotating said screw and thus move said not and slide back and forth on said base,

signal means including transducer means providing an electrical output displacement signal in response to the relative displacement of said slide on said base and having a relatively rotatable shaft and housing,

means directly connecting said shaft coaxially with said rotatable drive member to receive a rotary mechanical input motion therefrom,

means mounting said housing on said base for limited rotary adjustment with respect to said shaft thus modifying the relative rotation of said shaft and housing,

a compensating cam carried by said slide for movement therewith and having an adjustable cam surface and comprising,

a channeled member secured to said slide and extending beyond said slide parallel to the axis of said screw,

a plurality of discrete elements supported by said channel member and aligned rectilinearly along the direction of movement and carried for individual adjustment transverse said direction, said discrete elements having a length corresponding to the full travel of said slide and the effective length of said screw,

support means carried by said base supporting said channeled member throughout its full length during movement of said slide back and forth while enabling frictionless movement thereof of said channeled member,

a cam follower carried by said base and arranged to operatively engage said cam surface at a position generally opposite said support means and receive a limited rotary adjustment therefrom,

means connected to said cam follower at one end to receive rotary adjustment and connected at the opposite end to said housing imparting limited rotary corrective motion thereto to modify said relative rotation and thereby modify said electrical output signal,

and means for readily adjusting said discrete elements individually transverse said direction of movement to thus modify the effective compensating contour of said cam surface so that said rotary adjustment corrects the relative rotation of said shaft and housing and provides displacement signals accurately representative of the displacement between said members while compensating for errors in said drive and signal means.

References Cited UNITED STATES PATENTS 5/ 1960 Garber. 9/ 1964 Gitlin et a1.

US. Cl. X.R. 

